In a physiological study of the respiratory function a knowledge of the percentage of carbon dioxide in the expired air is quite helpful in determining the condition of the patient. The prior art shows several methods which have been used to accomplish this purpose. The first method employs an infra-red gas analyzer which consists of an infra-red source, a sample cell, a reference cell and a detector cell. The reference cell is divided into two compartments by a thin metal diaphragm which contains the same gas to be analyzed. Light passing through the sample cell differs in intensity from the light passing through the reference cell due to the absorption of the gas molecules in the sample cell. This results in a difference of pressure in the two compartments of the detector cell which moves the diaphragm and alters the capacitance between the diaphragm and the fixed plate in the detector cell.
A second method utilizes a mass spectrometer which accelerates and focuses gaseous ions (ionized by electrons emitted from a heated filament) by a combination of electric and magnetic fields. The traveling path or time-of-flight is determined by the pattern and strength of these fields and by the mass-to-charge ratio of the ions. Thus, ions of different molecular weight can be differentiated and their substance measured.
However, these two methods must utilize large, bulky and complicated devices to be effective. The present device, which utilizes the principle of sound velocity on the mean molecular weight of the component gas, promises a small-sized, more economic and less complicated device.